The present invention relates generally to data communication systems, and in particular to apparatus for interconnecting data terminal and data communication equipment in a serial binary data communication system.
In general terms, a data communication system consists of a group of functional units whose primary purpose is to transfer digital data between two or more locations in a reliable manner. Some systems transfer messages between remote terminals via one or more switching centers; other systems transmit data from numerous remote terminals to a central data processing facility.
With few exceptions, digital data communications systems generate and receive signals in serial bit form. The signals are typically generated as DC pulses with a negative voltage representing a binary 1, or a "marking" condition, and a positive voltage representing a binary 0, or a "spacing" condition. Although the signals may be transmitted in that form, in low speed systems for example, modern high speed systems convert the voltage pulses to another form for transmission. The latter function is performed by a modulation/demodulation unit referred to as a modem or data set.
Each end of a data communication system includes a unit for either generating or receiving data, or both. Such units, called data terminal equipment, include keyboard terminals, CRT display terminals, paper tape punches, computers, etc. Data is received from or transmitted to the terminal equipment by the previously-mentioned modem or data set, called data communication equipment in general terms. The connection between the data terminal and data set is referred to as an electrical interface.
The most common electrical interface for serial data communication and data terminal equipment in the United States is that specified in Electronic Industries Association (E.I.A.) Standard RS-232-C. The RS-232-C standard defines electrical characteristics of the interchange signals and associated circuitry, specifies pin assignments for a 25 pin interface connector, and functionally describes the various data, timing and control interface circuits.
Often there is a need to connect several terminals or the like to a common data set or modem. In the past this has been accomplished by "hubbing". For half duplex operation, hubbing involves connecting the terminals in parallel with the data set -- output-to-output and input-to-input -- through suitable leg circuits. All of the inputs and outputs are then connected to a common point, or hub. For full duplex operation, the terminals are connected in parallel, i.e., with their inputs all tied to one common point and their outputs to another. The common input and output points than are cross-connected to the data set output and input, respectively. Again, each unit is connected to the others through individual leg circuits.
Hubbing has a number of drawbacks, particularly in the full duplex arrangement. In that mode, for example, the terminals on the hub cannot communicate with each other, but only with the remote terminal. If more than a few terminals are involved, hubbing becomes disadvantageous because of the need for individual leg circuits for each data device. In addition to the expense involved in providing the circuits, system speed may need to be reduced so that all of the circuits remain synchronized.
It is, accordingly, a general object of the present invention to provide improved means for interconnecting serial data terminal and communication equipment without the drawbacks associated with prior art means.
A further general object is to provide data equipment interconnection apparatus that provides an electrically transparent interface between the connected equipment.
Another more specific object of the invention is to provide a multiple input-output port interface unit for bridging together a plurality of data terminals and data sets in a desired arrangement.
Still another object of the invention is to provide an interface bridge having three or more I-O ports conforming to the EIA RS-232-C interface standard.
Yet another object is to provide a multiport interface bridge capable of accepting signals from any one of three or more connected data units and transmitting them to the other units, any of which units may be a data terminal or a data set.
A further object of the invention is to provide an interface unit of the type just described that provides an option of returning signals to the originating unit.
Still a further object is to provide a multiport interface unit having means for indicating visually the receipt or transmission of data at each interface port.